The present invention relates in general to spectrophotometory and in particular to a method and apparatus for optically analyzing a specimen by using two light beams, wherein variations in ratio between a reference light intensity and a specimen light intensity in dependence on the wavelengths are previously stored in a memory and the measure signal is corrected by the signal read out from the memory in accordance with the wavelength of the measure signal.
As a means for measuring the spectral transmittance or reflectance of a specimen or sample, there has been known a spectrophotometer employing two light beams. The spectrophotometer of this type is adapted to produce a light intensity ratio I.sub.S /I.sub.R between a reference light intensity I.sub.R and a specimen transmitting light intensity I.sub.S. If the reference light intensity I.sub.RO is equal to the specimen light intensity I.sub.SO when no specimen is placed in a sample cell, the ratio I.sub.S /I.sub.R represents the transmittance or reflectance of the specimen. On the other hand, when I.sub.RO and I.sub.SO are not equal to each other, the quantity corresponding to the ratio I.sub.SO /I.sub.RO is utilized to correct the ratio I.sub.S /I.sub.R thereby to obtain the transmittance or reflectance of the specimen. In general, it is known that the ratio I.sub.SO /I.sub.RO is often subjected to variations in dependence upon the variable wavelength of the light beam employed in the spectrophotometory due to differences between optical characteristics of optical elements disposed on the reference light path and that of the specimen light path or the like causes. Accordingly, it is required to store previously the ratio I.sub.SO /I.sub.RO for every wavelength and correcting the measured ratio on the basis of the stored ratio I.sub.SO /I.sub.RO upon measuring the specimen.
With a view to attaining such correction, it has been hitherto known that values corresponding to the ratios between the reference light intensity I.sub.RO and the specimen light intensity I.sub.SO are previously sampled and stored in a memory as the wavelength is scanned, and that ratio the between the reference light intensity I.sub.R and the specimen transmitting light intensity I.sub.S obtained during the actual measurement of the specimen is corrected by the signal which is read out from the memory in accordance with the corresponding wavelength of the measuring light beam.
The known spectrophotometer of the above described arrangement has a disadvantage that when a volatile memory is used as the storage memory, the correcting values stored therein will be erased every time the power source of the spectrophotometer is disconnected to an external power source and therefore the operator cannot avoid the trouble of having to write the correcting values in the memory every time before the measurement of the specimen is to be conducted by connecting the spectrophotometer to the power source. Such difficulty can be obviated by using a non-volatile memory, which is however very expensive and impractical from the economical view point. On the other hand, because the ratio between the reference light intensity and the specimen light intensity will vary as time elapses, it is desirable to rewrite the stored contents of the memory once a day. In this connection, when the writing-in of the correcting value relies on the operator's judgement, there may arise such undesirable case where the correct write-in operation may not carried out due to the fact that the write-in operation is initiated before the whole system becomes in a stabilized state, involving erroneous results.